I was in Greece a few times; from March until May and from September until December; and from my point of view, spring and autumn are the best seasons for visiting Greece.
I prefer to avoid the peak seasons, the most crowded and hottest months.
Of course, you can visit Greece any time of a year, but I especially recommend spring and autumn. 🙂
I always recommend avoid July till middle August. All the rest is gooood 🙂 Winter in Greece is also lovely 🙂